Gas turbine engines operate by passing a volume of high energy gases through a plurality of stages of vanes and blades, each having an airfoil, in order to drive turbines to produce rotational shaft power. The shaft power is used to drive a compressor to provide compressed air to a combustion process to generate the high energy gases. Additionally, the shaft power is used to drive a generator for producing electricity, or to produce high momentum gases for producing thrust. In order to produce gases having sufficient energy to drive the compressor or generator, it is necessary to combust the fuel at elevated temperatures and to compress the air to elevated pressures, which again increases the temperature. Thus, the vanes and blades are subjected to extremely high temperatures, often times exceeding the melting point of the alloys comprising the airfoils.
In order to maintain gas turbine engine components, such as the airfoils and outer air seals disposed about the tips of the airfoils, at temperatures below their melting point, it is necessary to, among other things, cool the components with a supply of relatively cooler air, typically bleed from the compressor. The cooling air is directed into the component to provide impingement and film cooling. For example, cooling air is passed into the interior of the airfoil to remove heat from the alloy, and subsequently discharged through cooling holes to pass over the outer surface of the airfoil to prevent the hot gases from contacting the vane or blade directly. Various cooling air patterns and systems have been developed to ensure sufficient cooling of various portions of the components.
Typically, each airfoil includes a plurality of interior cooling channels that extend through the airfoil and receive the cooling air. The cooling channels typically extend straight through the airfoil from the inner diameter end to the outer diameter end such that the air passes out of the airfoil. The cooling channels are typically formed by dividers or partitions that extend between the pressure side and suction side. In other embodiments, a serpentine cooling channel extends axially through the airfoil while winding radially back and forth. Cooling holes are placed along the leading edge, trailing edge, pressure side and suction side of the airfoil to direct the interior cooling air out to the exterior surface of the airfoil for film cooling. In blade outer air seals, a similar cooling channel extends between an inner circumferential surface that seals against the blade tips and an outer circumferential surface that contains the cooling air. Holes are typically provided in the inner circumferential surface to bleed cooling air to the tips of the blades.
In order to improve cooling effectiveness, the cooling channels are typically provided with trip strips and pedestals to improve heat transfer from the component to the cooling air. Trip strips, which typically comprise small surface undulations on the airfoil walls, are used to promote local turbulence and increase cooling. Pedestals, which typically comprise cylindrical bodies extending between the channel walls, are used to provide partial blocking of the passageway to control flow. Various shapes, configurations and combinations of partitions, trip strips and pedestals have been used in an effort to increase turbulence and heat transfer from the component to the cooling air.
Sometimes, it is desirable to obtain different heat transfer characteristics at different radial or circumferential positions along the component, particularly in microcircuits comprising narrower channels located between more centrally located channels and the pressure side or suction side of an airfoil. The microcircuits can be further formed by the use of ribs that subdivide the channel into individual circuits. Trip strips can be positioned within the cooling channels to vary the heat transfer, but trip strips are difficult to position within microcircuits. Microcircuits are typically manufactured using a constant thickness sheet of refractory metal, thus fixing the width of the cooling channel. It has been proposed to use microcircuits having cooling channels of constant width that are tapered (decreasing in length between the leading and trailing edges) in the radial direction to decrease cross-sectional area and increase heat transfer properties at the tip of the blade, as is described in U.S. Publication No. 2010/0003142 to Piggush et al., which is assigned to United Technologies Corporation. However, large differences in the heat transfer coefficient are difficult to achieve without the ability to change the Mach number of the coolant fluid, which is typically done with some type of augmentation feature such as trip strips or pedestals. There is a continuing need to improve cooling of turbine components at different radial or circumferential positions of the cooling channels to increase the temperature to which the components can be exposed thereby increasing the overall efficiency of the gas turbine engine.